Most often, slow cabin heat warm up (assuming it eventually blows hot) is caused by old/dirty coolant, which will build up and reduce the available coolant passage through any part of the system (radiator, heater core, hoses, or thermostat).Seeing as winter has fully hit new england now I have been letting the a3 warm up in the driveway in the mornings. I noticed that after 10 mins or so the car was not warm at all so I started letting it idle for about 20 minutes. Even then the car is stone cold and the temp needle is still buried on the cold side. It's not until about 25-30 minutes that needle just begins to creep up a bit, and even then the heat from the vents is less than luke warm. I have noticed that when driving, having not let it idle in the driveway, it takes twice as long as my STI used to take to get heat. Does anyone else experience this or is it more likely that my thermostat is stuck open, not allowing the car to warm up quickly?
Does the coolant in the overflow reservoir look dirty? Have you had a coolant system flush within at least 4 years?